Chronic middle ear disease is the most frequent disease in otolaryngologic field. The studies on bacterial distribution and middle ear mucosal changes were made by many otologist but still there are many problems. The middle ear mucosal change and granulation tissue are believed to be great important in tympanoplasty and influence hearing mechanism. The bacteriological and histopathological studies were made on 80 cases of the chronic suppurative otitis media which treated at Dept. of otorhinolaryngology, National Medial Center during a period of Mar. 1970 through Aug. 1974 and the following conclusion were obtained. 1) Patients in this study were 44 males (55%) and 36 females (45%). 2) In general the most common pathologic organism were proteus species (34.2%) and staphylococcus (28.5%) in single or mixed infection. 3) Distribution of bacterial organism were observed staphylococcus (39.6%) in noncholesteatoma cases and proteus species (51%) in cholesteatoma cases. 4) The histopathological changes of the middle ear mucosa were classified in 4 types ; infiltrative type (65%), transitional type (7.5%), fibrous type (22.5%), and allergic type (5%). 5) The histopathological change in the case of cholesteatoma (39) was similar to that of all cases (80). 6) Bacterial distribution in each pathological type was seen more frequently in proteus in infiltrative and transitional type, staphylococcus in fibrous type and psuedomonas in allergic type, respectively. 7) Microscopic cyst formation was noted in 32 cases (40%) and were found more in infiltrative change (62.4%) predominantly. 8) Also cyst formation was more frequent in the cases of non-cholesteatoma (65.8%) than that of cholesteatoma (53.8%). 9) Poor vessel formations was observed in pathological tissue especially in cases where cholesteatomas and cysts were found.